At the start of a block

A node is at the start of a block of content if one of the following is true:

• the node is the first perceivable content inside this block; or
• the node is not perceivable content, is before the first perceivable content in this block and there is no other perceivable content between the node and the block.

In other words, the node is either the first perceivable content inside the block, any non-perceivable content before the block but after the previous perceivable content, or any non-perceivable content inside the block but before the first perceivable content in it.

Several nodes may be at the start of a block, especially if there are several non-perceivable content nodes next to each other.

Order ("first", "before", …) in this definition is tree order in the flat tree.